A sample that is an object whose quantum efficiency is to be measured, and a standard object having a known reflectance characteristic are each attached to a sample window provided in a plane mirror. Based on respective spectrums measured by a spectrometer in respective cases where the sample is attached and the standard object is attached, the quantum efficiency of the sample is measured. The plane of an opening of an observation window is made substantially coincident with the exposed surface of the sample or standard object, so that direct incidence, on the observation window, of the fluorescence generated from the sample receiving an excitation light and the excitation light reflected from sample is prevented.